Age definitely doesn't figure, i know people who work for SJ who are in their mid 30's and older. I don't think previous airlines make a difference either, there a quite a few crew who are ex Virgin cabin crew.
All i can say is if you really want it (and from what i can tell, they are a great company to work for)keep trying. 8 aircraft over the next 3 years means they are going to need lots of crew and if their is one thing i have learnt persistance definitely pays off! Good luck